The present invention relates to multiple-layer sheet materials and more particularly to a polymeric multiple-layer material particularly suited for use in making containers for food products to be cooked or reheated in ovens.
All-plastic and plastic-paper laminates for use in making packages or containers are known. For example, U.S. Pat. No. 2,956,723 discloses a sheet material for making containers. The material is a laminated film having at least one lamina formed of a unidirectionally molecularly oriented, linear crystalline polyalkene, such as polyethylene, and at least one other lamina comprising low density polyethylene. Similarly, U.S. Pat. No. 3,927,245 discloses a resin-coated carton blank having a fiberboard core, both surfaces of which are covered by ethylene polymer coatings of specified thickness and density. A third ethylene polymer coating for instance polyethylene terephthalate is applied to the ethylene polymer coating which is to be on the exterior of a carton to be constructed from the blank.
Copending application Ser. No. 702,018, filed July 2, 1976, and assigned to the assignee of the present invention, discloses an all-plastic material which includes a core layer of high density polyethylene and at least one surface layer of lower density polyethylene. The softening point of the core layer is higher than the softening point of such surface layer. A container may be formed or erected from this material by conventional heat sealing techniques carried out at temperatures lower than the softening point of the core layer but at or higher than the softening point of each surface layer. As long as the temperature during the heat sealing operation remains below the softening point of the core layer, the core layer preserves the structural integrity and shape of the material.
The materials disclosed in prior art references, such as the foregoing, are for the most part intended to be used in making liquid containers such as milk cartons and the like where the nonporous nature of the material is the critical property. However, a new market or use for such materials is developing.
Microwave ovens are becoming more popular with consumers because of the speed at which such ovens can cook or reheat food and because of the lower energy requirements of such ovens relative to conventional radiant energy ovens. Because microwave ovens cook or reheat food from the inside out, the food containers are not subjected to the same temperatures as they are in conventional ovens. Containers used in microwave ovens are heated only by heat conducted from the food being cooked. Since the temperature demands on the containers are lower, consideration has been given to the use of all-plastic or partially-plastic containers for pre-packed frozen foods in place of the relatively more-expensive containers in which frozen food products are now typically packaged.